One of the predominant theories of aging is the free radical theory of aging.. This theory proposes that aging is caused by damage to cellular molecules following exposure to reactive oxygen species (ROS). ROS have also been shown to be able to cause cancer, so a viable hypothesis for the cause of the increased incidence of cancer seen in aging organisms is damage induced, at least in part, by ROS. The levels of ROS in cells are due to their rates of generation and their removal by antioxidants. We propose to study both processes to further define the relationship between aging and cancer. In particular, we will study the role of superoxide dismutase (SOD) in cancer incidence in various model systems. SOD is one of the primary antioxidant enzymes that removes superoxide radical from biological systems. There are two main forms of this protein: copper- and zinc-containing superoxide dismutase (CuZnSOD, also known as SOD1) that is found in the cytoplasm and nucleus of mammalian cells and a manganese-containing superoxide dismutase (MnSOD, also known as SOD2) localized in mitochondria. Both forms of SOD have been found to play a role in both cancer and aging. Both forms are in general found to be lowered in activity in cancer cells and overexpression of both forms of SOD has been found to suppress the malignant phenotype. CuZnSOD knockout mice demonstrate greatly increased levels of spontaneous liver cancer and heterozygous MnSOD knockout mice show greatly increased levels of lymphoma. Flies with either MnSOD or CuZnSOD knocked out exhibit greatly reduced lifespans, while flies overexpressing these proteins showed increased lifespans. We propose to determine the effect of SOD modulation on cancer incidence in various in vitro and in vivo models of cancer as aging occurs. In this way, we should provide evidence for or against the role of SOD and ROS in aging-associated cancer.